If this is the case then it is fairly straightforward to do, simply follow these steps:
• Open up Microsoft Word on your computer
• Select 'new document'
• Place your full address on the top right of the page
• Put your boss' name and company address
Then you are ready to start the letter. You should commence by addressing your boss with Dear and then their name. Next you have to state what your letter is about and what you are asking; i.e. That you wish to seek sick leave from work for a certain time period or indefinitely, and why you feel it is appropriate to ask for such leave.
It would also be a good idea to attach a doctor's note if you have one, indicating your illness. You should print out your letter on good quality paper, insert into a new envelope and address it to your boss. Then you can either post it or drop it into the office.
You should get a response from your boss either by mail or phone call. Your boss should state whether the company is willing to grant you sick leave and what the relevant payment implications may be.